Hexagonal Boron Nitride: The Thinnest

Insulating Barrier to Microbial Corrosion

Govinda Chilkoor,

†

Sushma Priyanka Karanam,

‡

Shane Star,

†

Namita Shrestha,

†

Rajesh K. Sani,

Venkata K.K. Upadhyayula,

∥

Debjit Ghoshal,

⊥

Nikhil A. Koratkar,

#,¶

M. Meyyappan,

△

and Venkataramana Gadhamshetty*

,†,▲

†

Civil and Environmental Engineering,

‡

Materials and Metallurgical Engineering,

Chemical and Biological Engineering, and

▲

Surface Engineering Research Center, South Dakota School of Mines and Technology, 501 E. Saint Joseph Boulevard, Rapid City,

South Dakota 57701, United States

∥

Green Technologies and Environmental Economics Platform, Chemistry Department, Umea University, Umea, Sweden, 90187

⊥

Department of Chemical and Biological Engineering,

Department of Mechanical, Aerospace and Nuclear Engineering, and

Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York 12180-3590,

United States

△

Center for Nanotechnology, NASA Ames Research Center, Moﬀett Field, Mountain View, California 94035, United States

Supporting Information

ABSTRACT: We report the use of a single layer of two-

dimensional hexagonal boron nitride (SL-hBN) as the

thinnest insulating barrier to microbial corrosion induced

by the sulfate-reducing bacteria Desulfovibrio alaskensis G20.

We used electrochemical methods to assess the corrosion

resistance of SL-hBN on copper against the eﬀects of both

the planktonic and sessile forms of the sulfate-reducing

bacteria. Cyclic voltammetry results show that SL-hBN-Cu

is eﬀective in suppressing corrosion eﬀects of the

planktonic cells at potentials as high as 0.2 V (vs Ag/

AgCl). The peak anodic current for the SL-hBN coatings is

∼36 times lower than that of bare Cu. Linear polarization

resistance tests conﬁrm that the SL-hBN coatings serve as a barrier against corrosive eﬀects of the G20 bioﬁlm when

compared to bare Cu. The SL-hBN serves as an impermeable barrier to aggressive metabolites and oﬀers ∼91% corrosion

inhibition eﬃciency, which is comparable to much thicker commercial coatings such as polyaniline. In addition to

impermeability, the insulating nature of SL-hBN suppresses galvanic eﬀects and improves its ability to combat microbial

corrosion.

KEYWORDS: 2D coatings, hexagonal boron nitride, microbial corrosion, sulfate-reducing bacteria

icrobially induced corrosion (MIC) results in an

unanticipated attack on metals in seemingly benign

environments and threatens a range of multi-billion-

dollar industries including aviation, surface transportation, and

water infrastructure.

MIC accounts for 20−40% of the annual

corrosion costs including direct and indirect impacts, which

have been estimated to reach as high as $1 trillion.

MIC poses

a signiﬁcant ﬁnancial burden in the U.S. alone in the form of

total direct costs ($30 −$50 billion/year),

biocide require-

ments ($1.2 billion/year),

and direct costs in oil and gas

industries ($2 billion/year).

The U.S. annually spends nearly

$6 billion to combat MIC eﬀects of sulfate-reducing bacteria

(SRB) alone.

The SRBs secrete exopolymers on metal surfaces

and form a bioﬁlm to induce uniform corrosion or localized

pitting attack using one or more of the following mechanisms:

(i) disruption of passivating metal-oxide ﬁlms, (ii) altering

redox conditions at the metal−solution interface, (iii)

regeneration of the electron acceptors, (iv) production of

aggressive metabolites (e.g., sulﬁdes), and (v) depolarizing the

cathodic reactions.

Major corrosion mitigation practices including protective

coatings and impressed current cathodic protection tend to fail

under MIC conditions. For example, commercially available

polymer coatings (e.g., epoxy liners) are prone to biodegrada-

tion, and they exhibit poor adhesion toward metals under

aqueous conditions.

8−11

The thickness of commercial coatings

(∼50−1000 μm) can also disrupt the functionality (e.g.,

Received: August 31, 2017

Accepted: February 12, 2018

Published: February 12, 2018

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electrical conductivity, porosity, dimensional tolerances) of the

underlying metals. Furthermore, SRB can accelerate the

corrosion cells present under cathodic protection systems

through cathodic depolarization by consuming hydrogen.

Recent studies have demonstrated the corrosion resistance of

graphene-based materials

1,12−15

under both abiotic and MIC

conditions. The graphene coatings exhibit excellent chemical

inertness, ductility, hydro phobicity, impermeability, and

strength. Screening-level life cycle analysis has demonstrated

environmental beneﬁts of graphene coatings compared to zinc

coatings, under both aggressive atmospheric corrosion

and

MIC con ditions.

16,17

However, the nanoscale defects in

graphene coatings serve as cathodic sites for capturing and

reducing terminal electron acceptors and simultaneously

aggravating galvanic corrosion of the underlying metals.

Unlike graphene, the 2D hexagonal boron nitrided (hBN) can

be obtained as an insulating coating to suppress the galvanic

eﬀects. The 2D hBN materials have been reported to yield

high-performance coatings endowed with exceptional proper-

ties related to adhesion, barrier, impermeability, and

stability.

18−21

The hBN-coated metals (e.g., Cu and steel)

show oxidation resistance against corrosive eﬀects of high

temperatures (1100 °C)

and harsh chemicals.

18,23,24

2D hBN-

coated Cu has been shown to be an excellent barrier against

Gram-negative Escherichia coli.

In this paper, we explore the use of a single-layered hBN

coating as a thinnest insulating barrier to protect metals against

MIC eﬀects of aggressive sulfate-reducing conditions. We use

Desulfovibrio alaskensis G20 as the model for the SRB and Cu as

a technologically relevant metal. This study provides insight

into the performance of 2D hBN coating against the MIC

eﬀects of the G20 strain in the planktonic and sessile forms.

Here, we assess and quantify the long-term performance of 2D

hBN coating on a temporal scale that reﬂects Cu/bioﬁlm

dynamics in corrosion cells that are continuously operated

under a fed-batch mode. A series of alternate current (AC)

electrochemical impedance spectroscopy (EIS) tests, direct

current (DC) electrochemical methods such as potentiody-

namic polarization, cyclic voltammetry (CV), and linear

polarization resistance, and imaging techniques have been

used to investigate the underlying mechanisms that enable the

2D hBN coatings to protect metals against harsh MIC

conditions. Finally, we demonstrate that the single-layered

hBN coatings suppress galvanic corrosion eﬀects that are

typically encouraged by single-layered graphene (SLG) coat-

ings.

RESULTS AND DISCUSSION

Characterization of Atomic Layers of Hexagon al

Boron Nitride. The low-magni ﬁcation image in Figure 1a

Figure 1. SEM, optical image, TEM and Raman characterization of as grown single layer hBN (SL-hBN). (a) The line in lower magniﬁcation

SEM image (indicated with yellow arrow) represent a Cu grain boundary and the dark patches are the Cu domains. Scale bar: 100 μm. (b)

The smaller scale faint grey zones (indicated in red arrow) in higher magniﬁcation SEM image represent few layered hBN ﬁlms. Scale bar: 2

μm. (c) Optical image of SL-hBN (light indigo) transferred onto a SiO

/Si substrate (background color). Scale bar: 50 μm. (d) TEM image

showing sheets of SL-hBN. Scale bar: 50 nm. (e) TEM image of folded edge showing a single parallel line conﬁrming the presence of SL-hBN.

Scale bar: 2 nm. (f) Selected area electron diﬀraction pattern shows the crystalline and hexagonal nature of SL-hBN. (g) Typical Raman

spectra for SL-hBN.

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show a monolayer of conformal hBN coatings deposited over

Cu foils by chemical vapor deposition (CVD). The Cu grain

boundaries are indicated with yellow arrows and the copper

domains as dark patches (Figure 1a,b). The higher-magni ﬁca-

tion image in Figure 1b shows a monolayer of SL-hBN with

large area on the Cu foil. The occasional faint gray zones

suggest the presence of bilayers in certain regions. To further

conﬁrm the presence of the SL-hBN ﬁlm on Cu foil, the sample

was transferred onto the SiO

/Si substrate using a poly(methyl

methacrylate)-based transfer method. The light indigo shade in

Figure 1c indicates a monolayer of hBN, whereas the brown

patch indicates the underlying SiO

/Si substrate through a

topographical defect induced during the transfer. The trans-

mission electron microscopy (TEM) analysis reveals the

transparent, aggregated crystalline structure for the hBN

coating (Figure 1d). Furthermore, the TEM image shows a

folded edge with one line indicating the presence of a layer of

hBN (Figure 1e) on the copper foil with a corresponding

interlayer spacing of 3.3 Å. These values match the theoretical

estimates for distances between lattice planes for bulk hBN.

The selected area diﬀraction measurement shown in the TEM

image conﬁrm an atomic-scale chicken wire (hexagonal)

pattern of hBN on the SL-hBN-Cu (Figure 1f). Finally, the

Raman data for the Si/SiO

-transferred hBN ﬁlm analyzed with

Figure 2. SEM images after air annealing (200 °C, 8 h) for (a) Bare Cu showing 70% of oxidized surface (b) SL-hBN-Cu showing 30% of

oxidized surface. Note: White area indicates oxidized copper and red area indicates protected copper. SEM images after 30% H

exposure

(2 h) for (c) Bare Cu showing 71% of oxidized surface (d) SL-hBN-Cu showing 45% of oxidized surface. Note: Green area indicates oxidized

copper and gray area indicates protected copper. (e) Raman spectra of a transferred CVD hBN ﬁlm on SiO

/Si showing surface coverage of

∼1488 μm

for hBN (equivalent to ∼88% surface coverage). WITEC 300R Confocal Raman Imaging was used with a laser wavelength of 532

nm and 100× objective lens.

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the Lorentzian peak ﬁtting method

conﬁrm the Raman peak

at 1370 cm

−1

(Figure 1g) that matches a single layer of hBN

ﬁlm.

Identifying Inherent Defects in SL-hBN. We carried out

detailed oxidation experiments to identify the defect density of

SL-hBN coatings. The results from the oxidation experiments

are summarized here with further details below: (1) SL-hBN

coatings were able to protect the underlying copper substrates

in oxidizing environments; (2) SL-hBN coatings are charac-

terized by inherent defects; (3) Raman mapping tests

corroborate the speciﬁ c degree of surface coverage oﬀered by

the single layer of hBN coating. The oxidation experiments

were carried out by exposing the SL-hBN-Cu and bare Cu to

oxidizing environments induced by air annealing (∼200 °C, ∼8

h). Another set of oxidation experiments was carried out by

treating the samples with 30% H

for ∼2 h. The exposure

time determined from preliminary trials ensured aggressive

oxidizing environments without destroying the samples. Figure

2a,b presents the scanning electron microscopy (SEM) images

for the oxidized samples at the end of the air annealing

experiments (∼200 °C, ∼8 h). The oxidized surface is shown as

the white layer on the copper substrate (red background). A

quantitative analysis using ImageJ software revealed that the

percentage of oxidized area for SL-hBN-Cu and bare Cu was

∼30 and ∼70%, respectively. Exposure to H

revealed that

SL-hBN-Cu is susceptible to oxidation under aqueous

conditions (oxidized surface in green color and unaﬀected

copper in gray color) (Figure 2d). The H

experiments

revealed that the SL-hBN-Cu and bare Cu were oxidized by

∼45 and ∼71%, respectively (Figure 2c,d). These results

indicate that defects due to grain boundaries and uncoalesced

grains (measured in terms of the % copper oxidation) aﬀect the

surface coverage oﬀered by the SL-hBN coatings.

We carried out additional Raman mapping tests after

transferring the ﬁlms of SL-hBN-Cu on SiO

/Si specimens

(exposed area ∼1431 μm

)(Figure 2e). The Raman mapping

results indicate that hBN coatings were characterized with a

coverage area of ∼88%. Considering this situation, the

enhanced performance of the hBN coatings (i.e.,lower

corrosion rates) is attributable to its insulating nature and its

resultant ability to repress galvanic eﬀects (as discussed later).

Improving surface coverage is expected to further enhance the

ability of hBN coatings to combat MIC.

Crystallographic Orientation of Cu and the Eﬀect on

Defects in SL-hBN. The hBN ﬁlms grown using the CVD

process are prone to intercalation of corrosion molecules due to

the following two factors: (i) defects that develop due to

discontinuous formation of layers in the form of isolated islands

or domains and (ii) mismatch of copper and hBN orientation

due to polycrystalline copper which introduces diﬀerent

interfaces with hBN, allowing ions to intercalate between the

hBN ﬁlm and copper substrate.

27−29

Based on previous studies,

the lowest defect density for hBN growth on copper requires

the Cu surface to have a [111] direction.

30,31

The single-crystal

orientation of Cu helps to match the hexagonal lattice of hBN

to produce high-quality and large-scale conformal coating. It

eliminates the presence of numerous small-sized grains and

grain boundaries which are sources of coating degradation and

Figure 3. Electrochemical data within 24 h of exposure to sessile SRB-G20 medium. (a) Potentiodynamic polarization and (b) cyclic

voltammetry curves for bare Cu and SL-hBN-Cu. (c) Optical images of Bare Cu and SL-hBN-Cu after destructive test. Scale: 20 μm.

Potentiodynamic polarization measurements in a potential range of −300 to +1000 mV from open-circuit voltage. Cyclic voltammetry

measurements in the potential window of −700 to +200 mV (vs Ag/AgCl) for bare Cu; for SL-hBN, positive potential was extended up to

+500 mV. (d) Nyquist plot of Bare-Cu and SL-hBN-Cu.

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increased defect density. The copper foils used for growing

hBN have unfavorable crystallographic orientation (Figure

S1a,b), which indicates that the as-grown hBN ﬁlms are also

expected to be characterized by defects. To further understand

the Cu grain orientations in SL-hBN-Cu, we carried out

electron backscattering diﬀraction mapping (Figure S1b). The

mapping for SL-hBN (Figure S1b) was mono crystalline

Cu[100]. Future work should involve utilizing CVD substrates

with favorable crystallographic orientation in an eﬀort to reduce

defects and improve the surface coverage of hBN coatings.

High-Performance, Nanoscale hBN Coatings for

Microbially Induced Corrosion Resistance. Sulfate-reduc-

ing bacteria represent a diverse group of prokaryotes that are

known to accelerate corrosion of technologically relevant

metals. We used D. vibrio G20 as the model for Gram-negative

mesophilic SRB to evaluate the MIC resistance of SL-hBN-

coated Cu using the following three experiments: (i) abiotic

control; (ii) planktonic cell test (G20 cells in suspension); and

(iii) bioﬁlm test (G20 cells encapsulated in a biological ﬁlm).

Biotic vs Abiotic Corrosion under Neutral Conditions. As

discussed in the Supporting Information (Figures S2 and S3),

the lactate C media did not exert any corrosive eﬀects on Cu in

the absence of the G20 strain (i.e., abiotic conditions). For

example, the open-circuit potential (OCP) for the biotic tests

shifted to negative values (−750 mV vs Ag/AgCl) when

compared to that with abiotic tests (0 mV vs Ag/AgCl) (Figure

S2a). The DC tests show that the polarization resistance (R

)

for biotic tests was ∼67 times lower compared to its abiotic

counterpart (Figure S2b), and corrosion rates (Figure S2c )

were ∼4 times higher. The EIS analysis also suggests that the

charge transfer resistance for biotic tests was ∼6 times lower

when compared to that of the abiotic tests (see Bode plots in

Figure S3a,b). The EIS analysis for the abiotic and biotic tests

indicates diﬀerent sequences of corrosion mechanisms and

diﬀerent electrical equivalent circuit (EEC) for each case

(Figure S3c,d).

Planktonic Tests: Passivation Eﬀects of SL-hBN Coatings.

We conﬁrmed the passive nature of SL-hBN coating against the

MIC eﬀects of the planktonic cells in ﬁve diﬀerent ways, as

discussed below. First, we used potentiodynamic polarization

tests to observe the current response of the working electrode

after sweeping its applied potential away from the OCP

conditions. The nobler OCP value for the SL-hBN-Cu cell

(−670 mV vs Ag/AgCl) when compared to that of bare copper

(−735 mV vs Ag/AgCl) supports the passivation property of

the SL-hBN coating (Figure 3a). Lower current density in the

anodic branch indicates its higher corrosion resistance (Figure

3a). The single-layered hBN coating limits the corrosion

current to ∼6 mA/cm

, which is ∼4-fold lower compared to

that of bare Cu (∼27 mA/cm

)(Figure 3a). The anodic

polarization curve for the bare Cu is dominant at higher applied

potentials (starting from 0 V vs Ag/AgCl). Moreover, the

anodic branch for the bare Cu test registers a sharp increase in

current density (at 0 V vs Ag/AgCl), indicating the breakdown

of the passivation ﬁlm on the Cu surface and subsequent

copper dissolution. In contrast, the current density values for

the SL-hBN-Cu remain constant during the entire potential

range, conﬁrming its ability to minimize copper dissolution

even at higher potential scans.

Second, we ran CV tests for the bare Cu and SL-hBN-Cu

cells for four consecutive cycles to investigate the underlying

mechanisms for the Cu corrosion. The CV results conﬁrm that

the anodic current observed during the potential polarization

tests is due to Cu oxidation (Cu → Cu

+2e

−

). For example,

the CV test for the bare Cu cell yielded a sharp increase in the

current density when the potential approached +30 mV with an

oxidation peak at ∼144 mV vs Ag/AgCl (Figure 3b). This

oxidation peak can be attributed to Cu dissolution. The

reduction peak (−40 mV vs Ag/AgCl) during the negative scan

corresponds to the reduction of Cu

ions. Figure 3b provides

the evidence for the ability of the SL-hBN coating to suppress

the Cu dissolution. The peak anodic current (i

) for the SL-

hBN coating (∼25 μA/cm

)is∼36 times lower compared to

that of bare Cu (∼915 μA/cm

). This anodic peak is absent for

the SL-hBN-Cu cell at positive potentials (0−500 mV vs Ag/

AgCl).

Third, we used optical imaging to assess the degree of

corrosion attack on the underlying Cu surfaces after four cycles

of the CV runs. The CV test represents a destructive

electrochemical test. As shown in Figure 3c, the bare Cu

suﬀered a higher degree of debilitation, whereas the SL-hBN-

coated Cu stayed intact even after the four CV runs.

Fourth, the post-mortem analysis revealed that the hBN

coatings minimized the degree of pitting attack on the

underlying Cu surface. We cleaned the surfaces of the corroded

specimens (i.e., after the CV runs) with ∼10% sulfuric acid to

remove remnants of the bioﬁlm and corrosion products. The

bare Cu sample was characterized by an extremely rough

surface with an abundant number of micropits throughout the

sample area, whereas the surface of the SL-hBN sample

remained intact, exhibiting only minor surface roughness and

isolated pits (Figure 3c).

Finally, EIS analysis was used to discern the protection

mechanism oﬀered by SL-hBN coatings against the planktonic

cells. As shown in Figure 3d, the magnitude of the Nyquist arc

(i.e., polarization resistance (R

)) in the SL-hBN-Cu cell is 3-

fold higher than that of the bare Cu cell. The EEC ﬁtting

analysis conﬁrms that the overall corrosion resistance in the SL-

hBN-Cu cell (R

corr

= charge transfer resistance (R

) + pore

resistance (R

) = 52.78 kΩ·cm

)is∼49% higher than that of

the bare Cu (21.13 kΩ·cm

)(Table S1). The impedance

spectra in all cases follow a two-time constant model that is

connected in series with the solution resistance (R

)(Figure

S3d). The ﬁrst-time constant describes a pore resistance (R

which accounts for ionic or electron conductive pathways in

copper corrosion products or SL-hBN coating. The constant

phase element (Q

) d escribes the corresponding pore

capacitance and represents the ingress of aggressive electrolytes

into the porous layers of the coating present on the Cu surface.

The second-time constant describes the resistance to charge

transfer (R

) between Cu and the electrolyte and the

capacitance due to double-layer phenomenon (C

Bioﬁlm Tests: Microbial Corrosion Resistance of the SL-

hBN Coatings. We used the linear polarization resistance

method to investigate the eﬀectiveness of the hBN coatings

against MIC of the bioﬁlm. At the end of day 24, the R

oﬀered

by the SL-hBN coating (∼20463 Ω·cm

) was ∼9 times higher

than that of bare Cu (∼2373 Ω·cm

)(Figure 4a). Figure 4b

shows that the MIC rates in the SL-hBN cell were ∼67% lower

on day 1, ∼82% lower on day 15, and ∼87% lower on day 24.

The lower corrosion rates suggest the ability to minimize the

corrosion attack on the Cu surfaces. The corrosion current

corr

) and corrosion rates were determined using the Stern-

Geary equation (eq 1).

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2246

corr

(1)

The values for the R

were determined from the slope of the

current−potential curve at the open-circuit potential. The value

for the constant B was determined using eq 2.

ββ

2.3( )

(2)

The Tafel slopes for anodic (β

) and cathodic branches (β

)

were obtained from the Tafel plot. Finally, the corrosion rates

were calculated using eq 3.

=CR

(EW)

corr

(3)

where the constant K = 1.288 × 10

milli-inches was used to

obtain corrosion rates in milli-inches/year (mpy). EW, ρ, and A

are the equivalent weight (31.7 g), density (8.94 g/cm

), and

area (1 cm

) of the Cu specimen, respectively.

Barrier Properties of hBN Coatings. We used the EIS

test as a nonintrusive method for elucidating the reasons for the

MIC resistance of the SL-hBN coatings against eﬀects of the

G20 bioﬁlm. The hBN coatings exert a dominant electro-

chemical polarization resistance to underlying corrosion

processes relative to the bare Cu cell (Figure 4c). The EEC

analysis conﬁrms that the corrosion resistance (R

corr

)oﬀered by

the hBN coatings ( ∼27.22 k Ω·cm

)is∼10 times higher than

that by the bare Cu cell (∼2.53 kΩ·cm

)(Table S2). The value

of the Q

for the SL-hBN coating was converted to interfacial

capacitance (C

) using the parallel resistance method developed

in an earlier study

and using the following equation

32,33

−

CR Q

(1 )/ 1/

(4)

where C

is the capacitance of coating ﬁlm, R

is the pore

resistance, and n is the exponent in the constant phase element.

The following ﬁndings imply that the hBN coating oﬀers

capacitive impedance behavior and provides a physical barrier

to stop aggressive metabolites (e.g.,H

and HS

−

) from

contacting the Cu surface (Table S2).

The value of C

for

SL-hBN surface coating is an order of magnitude lower than

that of bare Cu. The value of C

is also ∼3.4 times lower

(Table S2). These ﬁndings indicate that the hBN coatings

reduce the surface roughness and minimize pathways that relay

ions into the underlying Cu surface.

The χ

test conﬁrms an

excellent goodness of ﬁt between the measured and predicted

values for the EIS data (goodness of ﬁ t=10

−4

; average residual

<1%) (Figure S4). The compliance of the EIS data with the K−

K relations is conﬁrmed by the excellent goodness of ﬁt (10

−3

using χ

value).

We used the R

corr

value obtained from the EIS analysis to

calculate the inhibition eﬃciency using eq 5. The inhibition

eﬃciency (IE%)

of the single-layered hBN coating was

determined to be as high as ∼91% (Table S2), which is on par

with a commercial coating such as polyaniline used in abiotic

environments (∼96%).

−

()

100

corr,coated corr,bare

corr,coated

(5)

The optical evidence further corroborates the barrier properties

of the hBN coatings against the MIC eﬀects of the bioﬁlm

(Figure 5). Both the bare Cu and SL-hBN-Cu surface

developed identical bioﬁlm properties related to uniformity

and thicknesses (Figure 5a,c). However, the bare Cu surface

was characterized by an apparent layer of black corrosion

deposits (Figure 5a), whereas the SL-hBN-Cu surface remained

clean even after 24 days of the MIC experiment (Figure 5c).

Further, the underlying bare Cu surface suﬀ ered from localized

attack by aggressive SRB metabolites (e.g., organic acids and

S), whereas the SL-hBN-Cu specimen faced only a minimal

degree of pitting (Figure 5b,d). As discussed below, the X-ray

diﬀraction (XRD) results conﬁrm that the SL-hBN coating

remained intact until the end of the MIC experiment. The SL-

hBN-coated Cu has also experienced a slight increase in the

corrosion rates during the linear polarization resistance tests

(Figure 4a,b), which is attributed to the defects in the CVD-

coated hBN coatings. Based on the ratio of R

values for the

bare Cu and SL-hBN-Cu,

the percentage of the uncoated area

on the SL-hBN-Cu surface was estimated to be 9%.

These ﬁndings demonstrate that the impermeable nature of

the SL-hBN coating prevents the sulfate reducing bacteria and

its aggressive metabolic products (acids, HS

−

) from contacting

the underlying Cu surface. The pore size of the 2D hBN (60

pm)

is smaller than the eﬀective ionic radii (r) of a range of

participating redox species such as Cu

(r = 73 pm) and Cu

= 77 pm),

aggressive metabolites such as HS

−

(r = 207

pm),

37,38

and terminal electron acceptors such as hydronium

ion (H

)(r = 99 pm).

As explained in the subsequent

Figure 4. DC and AC corrosion tests establish microbial corrosion

resistance of SL-hBN coatings. (a) Polarization resistance data, (b)

corrosion rates, and (c) Nyquist plots for bare Cu and SL-hBN on

day 24.

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2247

sections of this paper, the MIC resistance of the hBN coating is

also attributed to its insulating characteristics that disable the

cathodic reactions (e.g., proton reduction) and the subsequent

galvanic eﬀects.

Signatures of Microbial Corrosion. At the end of the

MIC experiments, the corroded specimens were subjected to

visual tests and a series of SEM, energy-dispersive spectroscopy

(EDS), and XRD examinations to investigate the signatures of

the MIC attack. Both the bare Cu and SL-hBN-Cu electrodes

were characterized by thick and homogeneous bioﬁlms of rod-

shaped cells that is a typical characteristic of the D. alaskensis

G20 strain (Figure 6a,b). The visual tests revealed dominant,

dark-gray crystals in corrosion deposits representing chalcocite

(Cu

S), which is a signature of the MIC attack on Cu

surfaces.

7,40,41

The EDS analysis of the bare Cu and SL-hBN-

Cu surface shows the peaks for sulfur along with other elements

including C, Cu, O, and P (Figure 6c,d). We attribute the O

and C peaks to the carboxy (C−O and CO) components of

extracellular polymeric (EPS) bioﬁlm.

The X-ray diﬀractogram

of the bare Cu surface conﬁrmed the presence of chalcocite

crystals (Figure 6e). The D. alaskensis G20 strain also aids in

formation of chalcocite and EPS compounds to develop a

defensive barrier against Cu toxicity.

The SL-hBN-Cu exhibits

a poor signal for the C peak. The intensity of the chalcolite

peak for the SL-hBN-Cu surface is also 3 orders of magnitude

lower compared to that of the bare Cu (Figure 6f). These

results suggest that D. alaskensis G20 uses copper as the

electron donor (eq 4) and the sulfate or proton as the electron

acceptor (eqs 6−8). It participates in a dissimilatory sulfate

reduction to generate corrosive hydrogen sulﬁde (HS

−

) species

that is responsible for the MIC attack

(eqs 7−9). The lactate

is oxidized

throug h the lactate dehydrogenase enzyme

pathways to produce acetate (eq 11).

Anodic Oxidation

→+

−

eCu Cu 2

(6)

Cathodic Reduction (in the cytoplasm)

++→+ +

−+− − −

O8H8HSOH3H

(7)

+→

+−

H2 H

(8)

++→ +

−+−

O4HHHS4H

(9)

Chalcocite Production

+→++

+−

−

Cu 2HS Cu S H S

(10)

→+++

−−+

−−

CH CHOHCOO SO 2H

2CH COO 2CO HS 2H

322

(11)

In light of the dissimilatory reduction mechanisms, the

following results conﬁrm that the SL-hBN coating decelerates

the metabolic activity of SRB, associated production of biogenic

sulﬁde, and subsequent MIC attack by D. alaskensis G20. The

high-performance liquid chromatography (HPLC) analysis

conﬁrms the role of hBN coatings in suppressing the metabolic

activities. The electrolyte in the SL-hBN-Cu cell registered ∼2-

fold lower values for the organic acids (acetic acid and

propionic acid) when compared to the bare Cu cell (Figure

S5a). The SL-hBN-Cu cell has also registered lower values for

sulﬁde (HS

−

) concentration throughout the temporal scale

(Figure S5b). For example, the HS

−

concentration for the SL-

hBN-Cu cell on day 7 (∼63 μg/L) was ∼ 1.4-fold lower than

that of the bare Cu cell (∼88 μg/L). The rate of increase in the

pH for the SL-hBN-Cu cell (7.2 to 7.8) was also lower than

that of the bare Cu cell (7.3 to 8.2) (Figure S5c). The lower

rate of pH decline indicates the ability of the hBN coatings to

retard one or all of the following: (i) conversion of sulfate (a

salt of strong acid) to sulﬁde (a salt of weak base), (ii)

conversion of lactic acid (pK

= 3.86) to acetic acid (pK

4.76),

and (iii) proton binding by the sulﬁdes.

Galvanic Corrosion Eﬀects of hBN Coatings Com-

pared with Those of Graphene and Bare Copper .

Galvanic corrosion occurs when two dissimilar metals are

coupled in a corrosive electrolyte. The metal with more positive

potential in a galvanic series will act as a cathode, and the metal

with negative potential acts as an anode. Corrosion occurs at

the interface of such galvanic couples. A coating such as

graphene (graphite) has more noble potential compared to

copper in a galvanic series.

A small defect such as pinhole,

cracks, or scratches in graphene coating will expose a small

anode (Cu) to large cathode (graphene), leading to a large

anodic current so as to balance the electron requirement of the

cathodic site. The areas with even such minor coating defects

are prone to localized corrosion, which will create a ripple eﬀect

propagating it to other sites in the metal surface. In fact, recent

studies on galvanic eﬀects of graphene described that grain

boundary defects in graphene coatings will accelerate localized

corrosion by forming galvanic couples.

45−48

To study and compare the galvanic eﬀects of graphene and

hBN coatings with bare copper, we carried out galvanic

corrosion tests using the ASTM G 71-81 standards. These tests

were carried out by conﬁguring the Gamry potentiostat in a

zero-resistance ammeter mode with transferred single-layer

graphene and SL-hBN ﬁlms on a SiO

/Si wafer as cathode, bare

Cu as anode, and the deﬁned lactate C medium along with the

planktonic cells of Desulfovibrio alaskensis G20 as the electro-

Figure 5. Optical images of bare Cu and SL-hBN-Cu after 12 days

of MIC experiment. Corroded bare copper (a) before and (b) after

cleaning. MIC-resistant SL-hBN-Cu (c) before and (d) after

cleaning. Scale bar: 200 μm.

ACS Nano Article

DOI: 10.1021/acsnano.7b06211

ACS Nano 2018, 12, 2242−2252

2248

lyte. The procedures for transferring the SLG and SL-hBN ﬁlms

on a SiO

/Si wafer are depicted in Figure S6. Figure S7 shows

the optical images of the transferred SLG and SL-hBN ﬁlms

onto SiO

/Si wafer. Figure 7 shows a three-electrode galvanic

corrosion cell (details in the Methods section) used in this

study. SiO

/Si was used as a control specimen to compare the

galvanic current obtained from Cu/SLG and Cu/SL-hBN

couples. The galvanic corrosion test was carried out within 24 h

of the exposure to Desulfovibrio alaskensis G20 lactate C

medium with a stable value for the open-circuit potential.

Figure 8a shows that the galvanic current density (I

) for Cu/

SLG (∼32.9 μA/cm

)is∼392 times higher compared to that

for Cu/SL-hBN (∼8.4 × 10

−2

μA/cm

). The higher galvanic

current density is directly proportional to higher corrosion rate.

The lower galvanic current density in Cu/SL-hBN when

compared to that of Cu/SLG clearly demonstrates that hBN

coatings are far less vulnerable to galvanic corrosion eﬀects. In

fact, the galvanic current and galvanic corrosion rate of the SL-

Figure 6. Microbial corrosion signatures. SEM images of G20 bioﬁlm formed on (a) bare Cu and (b) SL-hBN-Cu after 12 days of microbial

corrosion experiment. EDS data shows sulfur peaks for (c) bare Cu and (b) SL-hBN. XRD data showing formation of chalcolite compounds

on the surfaces of (e) bare Cu and (f) SL-hBN-Cu.

Figure 7. Corrosion cell for galvanic current a nd potential

measurement.

ACS Nano Article

DOI: 10.1021/acsnano.7b06211

ACS Nano 2018, 12, 2242−2252

2249

hBN are comparable to those of an insulating SiO

/Si surface

(Table 1), indicating its outstanding insulating characteristics.

The galvanic current density was converted to corrosion rates

in mills per year (Table 1 ) using the Faraday’s law as described

by procedure ASTM G102.

××KI

corrosion rate (mpy)

where K

is the Faraday constant (0.1288, mpy), I

is galvanic

corrosion density in μA/cm

, EW is the equivalent weight of

copper (31.7), and ρ is copper density (8.94 g/cm

The signiﬁcantly higher galvanic corrosion rates in the case of

Cu/SLG clearly demonstrates that graphene coating on copper

leads to galvanic problems. The electrode potential from the

galvanic couples’ can be used to determine the “higher”

cathodic nature of the metal.

The more positive the value of

potential in the galvanic couple, the higher its cathodic nature.

From Figure 8b, galvanic potential of Cu/SLG is higher (−744

mV) than that of Cu/SL-hBN (−792 mV), which we expect is

related to the diﬀerences in electrical conductivity of SLG and

SL-hBN. Moreover, the galvanic potential of SL-hBN is

comparable to that of an insulating SiO

/Si surface (Figure

8b). Since graphene acts more cathodic when coupled to

copper than hBN, Cu/SLG couple generates higher anodic

current, leading to greater extent of galvanic corrosion. This is

the reason why SL-hBN coatings reduce the galvanic corrosion

eﬀects by ∼400 times when compared to SLG coatings, as

revealed in our experiments. It should be noted that the

electron mobility of hBN (∼4923 cm

/Vs)

is ∼154-fold

lower than that of copper (∼32 cm

/Vs)

and ∼40 times

lower than that in graphene (∼2 × 10

/Vs).

The

insulating behavior of hBN is due to the diﬀerences in

electronegativity between the boron and nitrogen atoms, which

results in localization of π-electrons around the nitrogen

atoms.

CONCLUSION

This study demonstrates the ability of single-layered hBN to

passivate the negative eﬀects of microbially induced corrosion.

It is the thinnest insulating coating that combats the corroding

eﬀects of the aggressive metabolit es generated by the

dissimilatory sulfate reduction process. The MIC resistance of

the single-layered hBN coating emerges from the combined

eﬀects of its impermeable nature (barrier for electron acceptors,

redox species, and biogenic chemicals) and insulating character-

istics (suppresses cathodic reduction and galvanic eﬀects).

METHODS

A complete description of the experimental procedures can be found

in the Supporting Information, and only a brief account is given below.

The copper foils were coated with single-layer hexagonal boron nitride

(CVD-2X1-BN; 20 μm thick; Graphene Supermarket) via chemical

vapor deposition using the earlier procedures deﬁned in the

literature.

53,54

D. alaskensis was anaerobically grown in the lactate C

medium using the procedures described previously.

55,56

Microbial

corrosion experiments were carried out in a 400 mL single-

compartment corrosion cell consisting of three electrodes: a graphite

plate as a counter electrode, a Ag/AgCl as a reference electrode, and a

working electrode area of 1 cm

. We carried out abiotic tests,

planktonic cell tests, and bioﬁlm tests as described in the Supporting

Information.

Cyclic voltammetry, linear polarization resistance, potentiodynamic

polarization, and electrochemical impedance spectroscopy were used

to analyze the resistance of the coatings against MIC conditions. The

procedures reported previously

were used to transfer the SL-hBN

coatings from an underlying copper foil to the SiO

/Si substrate. The

samples were a nalyzed us ing SEM, TEM, XRD, and Raman

spectroscopy. To determine the organic, inorganic, anions, and

metal ions, we collected the samples of the electrolyte from the test

cells periodically using a sterile syringe ﬁlter with a pore diameter of

0.2 μm. Concentration of organic acids (lactic, acetic, and propionic

acid) was determined using a Shimadzu HPLC equipped with an

Aminex HPX-87H column (300 mm × 7.8 mm dimension), an SPD-

10A (UV−vis) detector, and 0.005 M H

as the mobile phase.

Concentration of aqueous sulﬁde was determined using a spectropho-

tometer following the USEPA methylene blue method.

The pH of

the electrolyte samples was measured using an Orion Star Benchtop

pH meter.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acsnano.7b06211.

Detailed methods section, ﬁgures (electrochemical tests

for bare Cu foil in sterile and D. alaskensis media; ﬁtted

data for Nyquist plot for bare Cu, SL-hBN; organic acid

concentration, sulﬁde and pH measurements for bare-

Cu-cell and SL-hBN-cell ; temporal proﬁles of R

, pore

and double layer capacitance), tables (electrical equiv-

alent circuit parameters for Nyquist plot conducted in

planktonic medium for bare Cu and SL-hBN-Cu; EEC

Figure 8. Temporal variation of (a) galvanic current I

(μA/cm

)

and (b) galvanic potential (mV) for galvanic couples: Cu/SiO

/Si,

Cu/SLG and Cu/SL-hBN in Desulfovibrio alaskensis G20 lactate C

medium. Cu is used as anode and SiO

/Si, SLG, and SL-hBN as

cathode. The anode to cathode area ratios for all the galvanic

couples are maintained at 1.7.

Table 1. Galvanic Corrosion Rates Based on Galvanic

Couple Current Density

galvanic

couple

galvanic current density

(μA/cm

)

galvanic corrosion rate

(mpy)

Cu/SiO

/Si 6.22 × 10

−3

2.8 × 10

−3

Cu/SLG 32.9 14.8

Cu/SL-hBN 8.4 × 10

−2

3.79 × 10

−2

ACS Nano Article

DOI: 10.1021/acsnano.7b06211

ACS Nano 2018, 12, 2242−2252

2250

parameters for Nyquist plot conducted in sessile medium

for bare Cu and SL-hBN-Cu), and references for the

methods section (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: venkata.gadhamshetty@sdsmt.edu.

ORCID

Nikhil A. Koratkar: 0000-0002-4080-3786

Venkataramana Gadhamshetty: 0000-0002-8418-3515

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

Funding support from National Science Foundation CAREER

Award (#1454102), National Aeronautics and Space Admin-

istration (NNX16AQ98A), and the South Dakota Board of

Regents u nder the auspices of the Surface Engineering

Research Center (SERC) is acknowledged.

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